Known loudspeaker systems typically comprise a driver and an enclosure. The components of the driver typically include a magnet (or more specifically, a magnet with a top plate and yoke), a voice coil, a sound radiating element (typically, a cone), suspensions, and a basket or frame. The cone may also be referred to as a radiator or diaphragm.
In known loudspeakers, the voice coil is rigidly attached to the cone, and the magnet is stationary, attached to the enclosure of the loudspeaker. The voice coil and magnet are arranged such that the voice coil is placed in the magnetic field of the magnet. In operation, current flows through the voice coil, which when placed in this magnetic field, causes the voice coil to move in response to an applied signal. As the voice coil and cone move as one entity, movement of the voice coil results in movement of the cone to radiate sound.
In the design of known loudspeakers, a driver with a given set of parameters is often assumed, and the best type of enclosure is then selected based on those parameters. Thiele-Small parameters are typically used in the categorization of loudspeakers, and these parameters are derived from basic physical parameters of a loudspeaker such as its cone mass, compliances, volumes, etc. Given these parameters, an appropriate enclosure for the loudspeaker can be constructed. It is also possible to define driver parameters based on desired external characteristics of the final loudspeaker product.
Although drivers used in known loudspeakers operate on the same basic concept, there is a wide range in driver size and power. Woofers, for example, are drivers that are designed to reproduce the lowest frequencies, or bass end of the audible sound spectrum. Subwoofers are a special type of speakers, typically designed to reproduce the lowest portion of the spectrum. Because woofers and subwoofers are specialized reproducers, their design maximizes their potential for reproducing the lowest frequencies. Therefore, they will typically be designed with cones that are suspended in such a way as to promote maximal back and forth motion.
Furthermore, at low frequencies, a loudspeaker produces a sound pressure proportional to the net output volume velocity from all openings in the loudspeaker enclosure. This requires compression and expansion of air within the enclosure. Accordingly, the number of openings and the internal structure of a given loudspeaker system can be used to define its type.
The sealed enclosure system is generally considered the simplest type of loudspeaker system. This loudspeaker system comprises a driver in a box, with no other openings. In this loudspeaker system, air is moved directly by the driver (e.g. cone). This loudspeaker system tends to be considered a low-efficiency loudspeaker system for a given box size and bass cutoff frequency. Such sealed enclosure systems are well known in the art.
The vented enclosure system is another example of a known loudspeaker system. A vented enclosure system comprises a driver having a primary sound radiating element such as a cone, and at least one secondary sound radiating element. The secondary sound radiating element of a vented enclosure system can be a vent or aperture in the enclosure, that provides a means for the rear output of the primary sound radiating element to contribute to the total output of the loudspeaker system, generally in a very narrow range of frequencies. In this case, the net output volume velocity of air is the sum of the volume velocity produced by the cone and the volume velocity produced by the vent. The vent is usually built as a tube having a suitable cross-sectional area and length. While a particular vented enclosure system can be designed with multiple drivers and multiple vents, a corresponding single driver/single vent loudspeaker system exhibiting equivalent performance characteristics can always be derived. Such vented enclosure systems are well known in the art.
Alternatively, the secondary sound radiating element of a vented enclosure system may be a passive radiator or passive cone. This loudspeaker system may be used when there is not enough room for a long vent in the enclosure, or when the level of noise generated by fast moving air in the vent is not acceptable. For the purposes of this specification and the claims, such a passive radiator system is also referred to as a vented enclosure system. Such enclosure systems incorporating passive radiators and/or cones are well known in the art.
Other types of loudspeaker systems comprising multiple cavities, drivers and vents are also known in the art. Some of them are built as band-pass loudspeaker systems.
Vented enclosure systems are generally considered to be more efficient than sealed enclosure systems, given the same bass cutoff frequency and size. Moreover, vented enclosure systems typically introduce relatively less distortion to reproduced signals. These properties have largely contributed to the broad popularity of vented enclosure systems.
Recently, there has been an increasing market demand for loudspeaker systems capable of reproducing very low frequencies at high sound pressure levels with minimal distortions, and having a relatively small enclosure size. Initially, it would seem that a loudspeaker in the form of a vented enclosure system should be an excellent choice.
However, the performance of a vented enclosure system depends on a number of factors, including the size of the enclosure, low frequency extension, and input power. A high sound pressure level requires a high output volume velocity of air in a vent of the vented enclosure system. Unfortunately, a small enclosure size may not permit the use of a vent with a large cross-sectional area. As a result, the linear velocity of air in a relatively narrow vent can reach high levels, and produce audible turbulences. These turbulences are particularly audible at the bottom corner of the low frequency range, where the sound of the loudspeaker is radiated mainly by the vent.
Alternatively, a passive radiator might be substituted for a vent. However, while this will eliminate air turbulences, it is at the cost of more expensive construction. Furthermore, the passive radiator may need to be as large or even larger than the active driver, which typically will require that the passive radiator be mounted on a separate surface on the loudspeaker enclosure. This restricts flexibility in the placement of such a loudspeaker in a room.
The total acoustic pressure produced by a vented enclosure system is the sum of the pressures produced by the primary and secondary sound radiating elements of the vented enclosure system. This can give rise to another problem not always appreciated by designers of loudspeakers. The primary and secondary sound radiating elements of a vented enclosure system may be spaced apart on a surface of the loudspeaker enclosure, or mounted on different surfaces of the loudspeaker enclosure, possibly even on opposite surfaces. These factors, as well as the placement of the loudspeaker in a room, can introduce relative phase and amplitude distortions to those pressures, thereby preventing their perfect addition, and lowering the efficiency of the loudspeaker at some frequencies. Some very high-powered loudspeaker systems are built as sealed enclosure systems, despite their lower efficiency, in order to avoid this problem.